In general, a capacitance type position detecting sensor in which position detection is carried out by detecting a change in capacitance needs a certain level of capacitance or higher (normally about 1 pF) that occurs between the position detecting sensor and a position indicator in order to detect the position of the position indicator with high accuracy. Thus, an input operation on the position detecting sensor is performed by a fingertip or a dedicated electrostatic pen having a relatively large area at a time of contact. That is, it is difficult from a viewpoint of detection sensitivity to detect a fine input position on the position detecting sensor by an electrostatic pen in the shape of a stylus having a fine tip.
Various techniques for solving this problem have conventionally been proposed. For example, Patent Document 1 (Japanese Patent Laid-Open No. Hei 7-295722) and Patent Document 2 (Japanese Patent Laid-Open No. Hei 8-272509) disclose a coordinate input device that has an alternating-current (AC) signal generator within a position indicator, and which detects the position of the position indicator by detecting a signal corresponding to the AC signal sent out from the position indicator in a position detecting sensor. In these Patent Documents 1 and 2, the AC signal sent out from the position indicator to the position detecting sensor has a large amplitude, thereby enabling position detection even with the position indicator formed by an electrostatic pen having a fine tip.
Specifically, Patent Document 1 shows a structure in which a signal oscillator 1 is provided within a signal pen 5, an AC signal of a large amplitude is generated by the signal oscillator 1, and the generated AC signal is applied between a stylus conductor 3 and a ring-shaped conductor 4 in the tip section of the signal pen 5. Patent Document 2 shows a structure in which a sine wave generator is provided in a stylus 6, and a sine wave AC signal is boosted by using a transformer and then supplied to a tip 2 of the stylus.
Patent Document 3 (Japanese Patent Laid-Open No. 2007-183809) shows the following structure. When a tip section 2 of a stylus input device 1 touches the surface of a capacitance type touch panel 51, a movable contact of a switching circuit 3 is switched to the side of a fixed contact 3a, and a phase comparator 5 is set in an operating state. As a result, an AC reference signal Vi of a frequency Fo, which signal is generated by a reference signal source 54 in a capacitance detection type coordinate input device 50, is received via the tip section 2, and then supplied to one input of the phase comparator 5 via the fixed contact 3a of the switching circuit 3. Because of this structure, a PLL circuit including the phase comparator 5 is phase-synchronized with the AC reference signal Vi, and outputs a signal of the same frequency as that of the AC reference signal Vi.
When the movable contact of the switching circuit 3 is switched to a fixed contact 3b, the phase comparator 5 is set in a non-operating state. However, the stylus input device 1 outputs a signal that is phase-synchronized with and having the same frequency as that of the AC reference signal Vi, which is generated in the reference signal source 54 in the capacitance detection type coordinate input device 50 due to a flywheel effect of the PLL circuit. A signal having an opposite phase from the output signal and an amplitude Vneg exceeding a power supply voltage Vcc is output via the tip section 2 of the stylus input device 1.